A variety of publications describe the participation of prolactin in the development and differentiation of keratinized structures such as hair, feathers, scales, tubercles and nuptial pads. The action of prolactin is in some systems stimulatory and in others inhibitory. It often synergises with other hormones. In an effort to gain better understanding of prolactin as a controlling factor in keratinization and as a synergist, its action on the nuptial pads of the newt will be examined under numerous experimental conditions in intact and hypophysectomized male animals. Its recently discovered potentialities in the female newt will also be explored. Preliminary observations will be made on the hormonal control of changes in the spermathecal epithelium of the female newt and the hedonic glands of the male newt. The apparently antagonistic interactions of prolactin and the thyroid hormone are also under investigation, particularly as they affect proliferation in the germitival layer of the epidermis in the newt.